1. Field of the Invention
The present invention relates to a light beam position controlling apparatus for an optical disk apparatus and an optical magnetic disk apparatus, which is adapted to optically record, reproduce, and erase information through application of light beams such as laser lights, etc. on a record medium.
2. Description of the Prior Art
Conventionally, in optical disk apparatus or optical magnetic disk apparatus, the record medium's face is displaced in the vertical direction, i.e., optical direction due to the movement of the disk face. The light beam spot is controlled in the disk vertical direction for adjustment so that the beam spot of the light beams might follow the displacement of the disk and be always focused on the medium face. Also, during the disk rotation, the information track portion on the disk is displaced in the lateral direction, namely, in the disk radius direction because of the eccentricity between the center of the tracks on the disk and the rotational center of the motor rotating the disk. Thus, the light beam position is controlled in the disk radial direction for adjustment so that the beam spot of the light beam may be always positioned on the information track by moving the beam spot of the light beam to follow the displacement of the information track on the disk. Also, position controlling, i.e., access controlling is performed on the beam spot at a high speed and with high accuracy to get the selectable track of the entire disk radius zone.
Generally, among the well known mechanisms for beam spot control, there is a mechanism in which the objective lens supported by two parallel-plate springs is driven vertically and laterally by an electromagnetic force; a mechanism which rotates the mirror by electromagnetic force to tilt the incident light axis of the objective lens thereby to laterally displace the beam spot position or a mechanism which drives an optical head, supported by a slide bearing, in the lateral direction by electromagnetic force.
The focus controlling operation is effected by a feedback controlling operation of sensing the relative displacement, i.e. focus error between the beam spot and disk medium surface by the method of passing the focus error signal through a phase lead compensating circuit, and thereafter feeding it into the focus actuator driving circuit to drive the focus actuator. Also, the radial controlling operation is performed by a feedback controlling operation of sensing the relative displacement, i.e., radial error between the information track on the optical disk and the beam spot by the method of passing the radial error signal through the phase lead compensating circuit, and thereafter feeding it into the radial actuator driving circuit to drive the radial actuator. The phase lead compensating circuit is a circuit for improving the stability of the feedback controlling system.
However, various problems exit in the mechanical apparatus. In the feedback controlling system composed of the above-described actuator and phase lead compensating circuit, a phase lag compensating circuit having break point frequencies between a disk rotation frequency, wherein the frequency component of displacement is largest, and the actuator resonance frequency is built-in in the feedback controlling system to improve the compression factor near the disk rotation frequency. However, as the damping factor of the actuator is small (that is, in general, the damping factor .zeta.&lt;0.5), the phase lag .theta. in the feedback controlling system with the phase lag compensating circuit built-in therein becomes smaller than (-180.degree.) near the actuator resonance frequency. Thus, the operation is so unstable that the above-described phase lag compensating circuit cannot be built-in to the feedback controlling system, or if it has been built-in, the results have to be controlled by making the gain extremely small. As a result, the compression factor of the feedback controlling system becomes lower, so that the desired servo performance cannot be provided.
Also, in a mechanism wherein the objective lens is supported with the above-described two parallel springs or a mechanism for rotating the mirror, the entire controlling mechanism becomes too large as compared with the optical head for moving the beam spot across the entire disk radius zone. Also, in the mechanism wherein the optical head is supported by a slide bearing, the holding force of the moving-part with respect to the radial direction is weak and the mechanism is weak against the disturbance oscillation applied upon the optical disk apparatus. Also, in the mechanism supported by the slide bearing, the beam spot position controlling operation with a precision on the order of sub micron or less cannot be performed due to influences by the stick slip or the like of the slide bearing portion.
A method of performing parallel controlling operation by both mechanisms is considered from these disadvantages to compensate for the respective disadvantages of a mechanism for supporting the objective lens with two parallel springs or a mechanism (hereinafter referred to as radial actuator mechanism) for tilting the incident optical axis of the objective lens through rotation of the mirror with the use of electromagnetic force to laterally displace the beam spot and a mechanism (hereinafter referred to as linear mechanism) for driving the optical head, supported by a slide bearing, with the use of the electromagnetic force. But both mechanisms are mutually negatively influenced with respect to the disturbance oscillation applied upon the optical disk apparatus in the simple driving operation of both the mechanisms, thus resulting in the oscillation increasing towards the unstable condition. As the driving force of the linear motor is not applied directly upon the objective lens supported by the radial actuator mechanism, phase lag is caused in the displacement of the objective lens from the displacement of the optical head, so that the stable parallel controlling operation cannot be performed.
Also, at the switching from the access control to the radial control or from the jump control to the radial control, the setting condition of the light beam position immediately after the servo loop of the feed-back controlling system has been closed was extremely unstable, because the position of the information track always moves and the initial value of the radial error is thus greatly changed. To improve the stability, there is a method of storing the radial driving current synchronously with the disk rotation in the closed loop condition of the feedback controlling system, adding a memory signal to the radial actuator during the closed loop and the open loop of the radial control system to reduce the relative displacement, i.e., radial error between the information track on the disk and the beam spot position thereby to reduce the instability of the radial control setting condition.
However, the radial driving current also had a noise component, a component caused by the shape of the individual information track, and a high-frequency component for making the closed loop servo condition stable, except for the frequency component resultant from the eccentric condition of the information track. Thus, if the radial driving current is stored as it is, and is fed to the radial actuator, the radial setting condition does not become stable as expected, thus resulting in instability due to the above-described components.